- [Instructor] What you see depicted
here is known as a pedigree.
And a pedigree is a way of analyzing
the inheritance patterns of a trait within a family.
And it can be useful to understand more about that trait,
maybe to make some insights about
the genetics of that trait, and it's a way
to think about what's happened in the past in a family,
and then maybe we can help get some probabilities
or get some understanding of what
might happen in the future.
But let's just break it down, so we understand
all of the different parts of a pedigree.
So big picture, a circle represents a female in the family.
And a square represents a male in the family.
If either the circle or the square is colored in,
like we see right over here, or we see right over here,
that means that that person exhibits the trait.
And the trait of this particular pedigree
is for is the trait of freckles.
And whether or not we're colored in, that is a phenotype.
That is an observed trait.
You just look at the person and say
do they have freckles or not.
This male right over here does not have freckles.
This female right over here does have freckles.
This male right over here does have freckles.
But once again, this phenotype,
that's the observed characteristic,
we don't know exactly the genotype just from this.
Although we might be able to infer some ideas
about the genotype by seeing how the phenotype patterns
spread from generation to generation.
But just going back to understanding the pedigree itself.
When you have a horizontal line
connecting a male and a female, it's called a marriage line.
And it means that they are coupling up
and they are reproducing.
They don't necessarily, I guess, have to be married.
It's more that they reproduce.
So this male and this female have reproduced.
And then this vertical line that goes
from that horizontal line,
that's known as the line of descent.
So it's going from that first generation
to the second generation.
And so all of the people connected to that vertical line
at the bottom of the vertical line,
these are their children.
All the people who are directly connected to this.
And this is known as a sibling line.
So, this person, this person and that person,
they are all brothers and sisters.
You can see that generation one,
they had two daughters, one of whom exhibited freckles,
and one son, who had freckles.
Now these other people in generation two,
these aren't children of the first generation.
These are people, you could say,
who are brought into the family.
They either married into the family,
or you could just say they reproduced with the children.
But that's what's happening with generation two.
And so you can see, you have another
what's known as a marriage line,
but I guess it could be a coupling line, a line of descent,
and then they have two daughters in this case.
So it turns out that freckles
is an autosomal dominant trait.
And I will explain what that means in a second.
Autosomal
dominant,
dominant trait.
Autosomal means that it is a trait associated
with one of the non-sex chromosomes.
We have 23 pairs of chromosomes as humans.
One of those pairs determines our sex,
the other ones don't determine our sex.
So this means that freckles is associated
with one of the 22 pairs of chromosomes
that do not determine your sex.
And dominant means that in general
you get two versions of a gene.
One from your mom, and one from your father.
And each of those versions of the gene for say
whether or not you have freckles is known as an allele.
And if a trait is autosomal dominant,
that means that if you have even just one allele
for that trait, that you are going to exhibit the trait.
And so, for example, if the alleles for freckles,
if capital F is you have freckles,
it's dominant, so you write it in capital,
and lowercase F means that you don't have freckles,
if someone has a genotype of capital F, capital F,
or capital F, lowercase F, or, I guess we could say
lowercase F, capital F if we wanted to think about order.
Which one you're getting from which parent.
Well then in all of these situations,
they would exhibit the phenotype of freckles.
And we talk about genotype and phenotype in other videos.
And of course, the only genotype in which
they would not exhibit freckles
would be having both of the recessive traits.
So with this out of the way, what can we infer
about the genotype of this individual right over here?
And this individual will sometimes be
referred to as individual I-2.
They're in generation one,
and they are individual two in generation one.
Well, we know that they exhibit freckles.
So we know for sure that since the phenotype is freckles,
they're in one of these scenarios.
And these two, these two are frankly equivalent.
So this person, we know that they're either
capital F, capital F, so that would be homozygous dominant.
Or they're capital F, lowercase F.
Which are, they are a heterozygote.
Well, let's first think about the situation
if they are homozygous dominant.
If they are homozygous dominant,
that means to every one of her children,
she would give one of these alleles.
And either of those are dominant.
And so that would mean if this were the case,
that all of her children would get
at least on dominant allele.
Well, if they all get at least one dominant allele,
then all of her children would have freckles.
But we're seeing a situation where
all of her children do not have freckles.
Two of her three children had freckles.
So this cannot be the case.
So this individual right over here
must be heterozygous for freckles.
So they must be capital F, lowercase F.
And what do we know about this individual right over here,
the person that she coupled up with?
Well, they don't exhibit the phenotype for freckles.
So they must be lowercase F, lowercase F.
And then from that, you might be able
to intuit a lot more about that.
And we'll do that in future worked examples.